(1) Field of the Invention
The present invention relates to a high frequency oscillation type proximity switch and more particularly to a proximity switch whose response speed at the start of oscillation is improved.
(2) Description of the Prior Art
A high frequency oscillation type proximity switch has a detecting coil installed in a detecting head, said detecting coil being used as an oscillating coil to form an oscillating circuit, an object being detected on the basis of a decrease in the oscillation output. The oscillating circuit used in proximity switches is usually the Hartley oscillating circuit, and the speed at the stoppage of oscillation is changed by adjusting the shape and the number of turns of the oscillating coil and other circuit constants. However, if an approaching object approaches to increase the conductance of the detecting coil, the oscillation will stop sooner or later. Therefore, it may be considered that the response speed of a proximity switch is the total time of the start and stop speeds of oscillation. Generally, the oscillating circuit has a low oscillation rise (start) speed and a very high stop speed. The larger the shape of the detecting coil, the lower the response speed. For example, while a coil with a coil diameter of about 12 mm has a response speed of several hundred KHz, a coil with a coil diameter of about 40 mm has a response speed of several hundred Hz. Thus, conventional proximity switches have the problem that if the coil diameter is large the response speed is low and hence the approach of an object whose rate of change is high cannot be accurately detected.
Further, in an environment where a large current of several ten thousand A flows to produce an intense ac magnetic field as in a resistance welder, the ferrite core of the detecting coil is saturated to increase the detecting coil loss to the extent of stopping the oscillation. Thus, there has been a problem that in such environment it is impossible to use a high frequency oscillation type proximity switch.